Technical Field
The present invention relates to management of energy storage systems, and, more particularly, to a modular multilevel converter and control framework for management of hybrid energy storage systems.
Description of the Related Art
With high penetrations of Photovoltaic (PV) systems in the power grid, short-term, high-frequency fluctuation of the PV output power during unpredictable weather variations is increasingly becoming a concern. To support the penetration of renewable energy generation in the power grid and to provide ancillary functions for system operation (e.g., compensate for the fluctuation), the demand for energy storage systems (ESSes), which may store a large amount of energy and provide high charging/discharging power as needed, has increased. However, single type energy storage elements cannot store a large amount of energy and provide high charging/discharging power, and as such, hybrid ESSs (HESSs) have been employed to utilize the advantages of different energy storage elements to provide a solution for this issue.
Batteries have a relatively large energy density and UCs have a large power density. By combining them together, the HESS can satisfy all the power requirements to smooth the PV output power. Furthermore, the UC can alleviate the high power burden on the battery, extend the battery lifetime, and reduce the size and power loss of the battery.
Conventional HESSs with a battery and a UC generally employ a two-stage configuration, which includes a dc/dc converter and a dc/ac inverter. The addition of a dc/dc converter increases the system cost, and also introduces extra power loss. Furthermore, with the increased power and energy ratings of the HESS for utility-level applications, the power semiconductor devices and passive components in these topologies may become unsuitable to handle the high voltage and current presented in the system.